Everything! I mean everything! There's fakes of literally everything. Including items you wouldn't think would be profitable to fake.

Books, CD, electronics, baby products, stationary, personal care products including condoms, toys,...

Take a look at this cup. Would you know if Amazon sent you a fake if it's your only Yeti cup? http://www.wideopenspaces.com/fake-yeti-ramblers-on-the-rise...

Many/Most of the time it's going to be difficult to tell unless you have the real version already and do side by side comparison.